In 1951 William C. Moog, Jr. developed the first two-stage servovalve using a frictionless pilot stage. A flapper and nozzle variable
orifice was used in conjunction with a fixed orifice to drive a second-stage
spool in a three-way mode. The flapper-nozzlevalve was driven by a
torque motor, and spool position was achieved by a spring acting directly on the spool.

The advantages of thus construction were an appreciable reduction in valve threshold, and a high dynamic response because of the lower mass of the first-stage parts. Frequency response on the order of 90” at 100 Hz was possible allowing for the use of servovalves in high gain positron servos.